4,4′-difluorobenzophenone (4,4′-DFBP) is a well known chemical intermediate having the following chemical formula:

4,4′-DFBP is known to be useful in the preparation of, e.g., PAEK polymers such as PEEK and PEK. PAEK polymers are a well known class of engineering polymers useful in various fields of endeavour. Processes for preparing PAEK polymers, including those using 4,4′-DFBP, can be found in, e.g., U.S. Pat. Nos. 3,953,400, 3,956,240, 3,928,295, and 4,176,222, all incorporated herein by reference. Generally, PAEK polymers are prepared by aromatic nucleophilic substitution. For example, p-hydroquinone, commonly referred to as “hydroquinone”, a bisphenol, etc. can be used as a nucleophilic component which is deprotonated with a base such as NaOH, Na2CO3 or K2CO3 to form a nucleophile that then reacts with, e.g., a dihalobenzophenone such as 4,4′-DFBP to form a PAEK polymer via nucleophilic substitution, with the fluorine atoms of the 4,4′-DFBP acting as leaving groups.
It is generally known that purified starting materials are preferred in the chemical synthesis of complex molecules, and this is true for monomers used in the synthesis of PAEK polymers. For example, WO2007/144610 and WO2007/144615 describe the use of monomers having a purity of at least 99.7 area %, including 99.9 area % (as measured by gas chromatography), as providing improved melt flow index in the product polymer. It should be noted that a material that is 99.9% pure contains 1000 ppm of one or more impurities. However, these references remain silent on the nature and amount of specific impurities to be avoided. In addition, this measurement by area % leads only to a general purity level of the monomers and is nonspecific with regard to the type and amount of specific impurities to be avoided.
Common impurities of 4,4′-difluorobenzophenone are for example other positional isomers (mainly the 3, 4′ and 2, 4′ isomers), coloured impurities and polymeric by-products as described in U.S. Pat. No. 5,777,172.
Semi-crystalline poly(aryl ether ketone)s exhibit interesting properties as compared to their amorphous counterparts including, notably, excellent chemical resistance and good mechanical properties over a large temperature range. Ultimate mechanical properties of semi-crystalline resins are in particular linked to the crystallinity level. A high level of crystallinity is thus important to maintain these properties. Another important property of PAEK polymers is their melt stability.
There is a long felt need for PAEK polymer having improved chemical resistance and mechanical properties over a large temperature range, and therefore PAEK polymer with improved crystallinity and/or melt stability are needed.